Numerous efforts have been advanced over the years to combat the abrasive wear and tear of items subject to abrasive contact with hard surfaces, or with moving matter such as water or flying debris. In particular, a number of sheet materials have been developed to wrap-around items to be protected or to cover exposed surfaces. Other types of sheet material are directed at preventing harmful exposure to items being stored or transported in large containers.
One application of such materials is in air cushion transport vehicles and air flotation devices of the type in commercial use today. These vehicles and devices generally operate on the principle of providing a downwardly directed airflow against a ground or water surface. This downwardly directed airflow results in a lifting force, creating a “hover” or “ground effect”. An example of a vehicle of this type is an air cushion vehicle known as the HOVERCRAFT™. While there are many types of these vehicles, they each operate under the same general principles. A system is generally employed to provide an air cushion that supports a load, i.e., the weight of the vehicle and its cargo, just above ground or water level. This system typically comprises multiple high-volume, high-pressure fans that effectively pressurize the air held beneath the vehicle with enough force to lift the vehicle just above the surface over which the vehicle is positioned. A separate motive source, comprising fans or turbines, then propels the vehicle over the surface of the ground or water.
Air cushion vehicles typically include a skirt around the base, or rigid support frame, of the vehicle to enclose and form the air cushion. One portion of the skirt forms an inflatable bladder, or bag member, that is stiffened when inflated, but yet is open at the bottom. The skirt may also include a plurality of contiguous skirt elements, known as “fingers”, depending downwardly from the bladder. The fingers provide auxiliary bladders and help direct the pressurized air beneath the frame of the vehicle.
Skirts have conventionally been made of reinforced (Nylon) rubber, both natural and synthetic. A particular problem, however, with air cushion vehicle skirts is their susceptibility to damage and rapid deterioration due to constant impingement by water or foreign objects. When these vehicles are operated over land, the bottom edges of the skirts are subjected to severe abrasion and wear rapidly. Further, when used in a salt-water environment, rapid deterioration of the skirt material necessitates frequent replacement of the skirt after only a few operating hours, and at substantial costs.
Numerous attempts have been made to construct more durable, longer-lasting skirts, but these have proven unsatisfactory for one reason or another. In one skirt construction, apertured flexible sheets formed from a reinforcing fabric with an elastomeric protective material have been tried. In another, a molded seamless skirt has been formed from a high strength woven aramid (KEVLAR® brand) fabric, but based upon experimentation, KEVLAR® cannot withstand salt-water attack and ultraviolet light, and wicks when exposed to water. Steel wire has also been woven into some conventional fabric constructions, but has not resulted in any appreciable reduction in wear or longevity of the skirt.
At present, the average skirt installed on air cushion vehicles operated by the United States Navy must be replaced at least every 100 to 200 operating hours. Additionally, the fabric conventionally used in such air cushion vehicles is formed of a soft rubber composition that weighs at least 80 ounces (2240 grams) per square yard. One serious problem with this construction is that it has a low resistivity to abrasion. When subjected to the standardized Taber Abrasion Resistance Testing, the rubber assembly rapidly exhibited loss of base material. For example, in representative testing, over 37 percent (weight) of the rubber skirt material was lost after 40,000 abrasion cycles. As a result, these skirts must be replaced frequently which adversely affects the operational readiness of these military vehicles.
A recent development by one of the applicants of the present invention is a lightweight skirt assembly that comprises a sheet material formed of woven ultra-high molecular weight polyethylene (UHMWPE) that is coated with an abrasion resistant thermoplastic film such as ethylene vinyl acetate (EVA). This construction, when subjected to the same abrasion resistance testing, lost only about 22 percent of its total weight. While this construction provides a substantial increase in the service life of skirt assemblies for air cushion vehicles, it has some shortcomings. For instance, SPECTRA® yarns and fabrics, formed from a polymer of ultra-high weight average molecular weight polyethylene, have little memory; i.e., when a fabric woven from this material is bent or rolled, it tends to maintain that shape until manipulated into a different shape. Thus, when used for air cushion vehicle skirts, the skirts tend to roll up and retain their inflated shapes, even after being deflated. Further, the thermoplastic film tends to abrade from the lower portion of the skirt over time, exposing the woven fabric, which is aesthetically undesirable.